1. Field of the Invention
This invention relates generally to seismic sensors and more particularly to a gimbal geophone.
2. Description of the Related Art
In seismic exploration, a seismic source is used to introduce a seismic signal into the earth. The seismic signal propagates through the earth in the form of a spherical wave front. As the wavefront impinges upon the substrata layers (structural changes), a portion of the wave front is reflected back to the earth's surface. The reflected waves are recorded by a plurality of sensors and the recorded data is processed to obtain information about the earth's subsurface. In seismic exploration on land, shallow water and marsh lands, geophones are typically used as sensors.
Geophones are accelerometers, in that they contain a suspended mass and are designed to detect the movement of such mass. Such geophones require vertical or near vertical placement (orientation) for optimal results. Geophone manufacturers specify a maximum tilt angle (typically about 30.degree.-35.degree.) above which the geophone provides no useful information. In land surveys, geophones are implanted into the earth so that they remain near vertical during seismic survey. For shallow waters or marsh lands, gimbal geophones have been designed which provide a single degree of rotation or freedom. The geophone is placed in a sealed housing wherein it can rotate about a longitudinal or center axis. A plurality of such gimbal geophones are attached to a cable, commonly referred to in the art as the "bay cable." The bay cable is placed along a desired pattern on the earth's surface. The earth's surface, however, may have a slope greater than the maximum tilt angle of the geophone or there may be under water obstacles, such as reefs, man-made objects and the like. In such cases, the geophones may get placed with a tilt angle greater than the maximum tilt angle of the geophone, which is highly undesirable. Some geophone manufacturers provide separate geophones for use in horizontal and vertical configurations. This is highly undesirable because it requires the surveyor to know configuration of the earth's surface under water or marsh and configure the cable with different types of geophone. Also, it requires maintaining inventory of two types of geophones, which increases the operational cost and is thus undesirable.
It is therefore highly desirable to have a geophone which is (a) completely orientation-insensitive, i.e., insensitive in all directions within the tilt tolerance of the geophone, (b) relatively easy to manufacture and (c) does not cost substantially more than the currently available one direction gimbal geophones.
The present invention addresses the above-noted problems and provides a gimbal geophone which is completely orientation-insensitive.